Skip to main content

Full text of "The Cambridge Natural History"

See other formats


fossa, that it cannot be regarded as anything more than a mark
of specialisation. It is, in fact, the case that the Macraucheniidae
are in many points specialised, while retaining many primitive
features of structure.
The chief primitive features are : the non-alternating positions
of the wrist- and ankle-bones; these, of course, interlock in the
Perissodactyles of to-day and in many extinct families. Then
the absence of a diastema in the tooth series, coupled with the
presence in MacraucJienia, of a complete dentition. The small
brain may be referred to the same category. Macrauchenia
must have been a strange -looking animal. It walked upon
three toes on each limb; the skull was Horse -like in general
form, but the nostrils are removed to a point about as far back
as in the Whales or nearly so, the nasal bones being correspond-
ingly reduced. This it is thought argues a proboscis. The
humerus is particularly compared by Burmeister1 to that of a
Horse. The radius and ulna though both well developed are
fused. The neck is long, and, as in the Camel, the vertebral
arteries run inside the neural arches. Since the fore-legs seem
to have been rather longer than the hind-legs, though only very
slightly, and the neck was long, the animal may have presented
some likeness to the Giraffe. It is interesting to note that in the
proportions of humerus to ulna this animal is more Lama -like
than Horse-like. On the other hand, the proportions of femur
to tibia are more Horse-like. The remains of the creature are
limited to South America, and to quite superficial deposits. It
is evidently a specialised type, and has pursued a course parallel
fco that of the Horse. Much nearer to the Horse however, but
apparently by convergence only, is the genus TJioatherium,
usually placed in a separate family, the Protorotlieriidae. In this
creature, which has many archaic characters, the toes are reduced
to one in each foot. In an allied form, ProtorotJieriutn, we have
the two lateral toes diminishing just as in d.nchithe'rium.
3 tf. Ada, Amd. Caes. Zeop, Oar. xxvii. 1885, p. 238.